Nichia Corporation, Japan and Lumi LED, U.S.A. are going ahead in the fields of the development and production of blue and white light emitting diodes and laser diodes using GaN-based compound semiconductors. Recently, various high-luminance light emitting element structures to be applied to the fields of illumination such as household fluorescent lamps and LCD (Liquid Crystal Display) backlights have been proposed and produced. GaN-based materials have well exhibited their possibilities not only as optical elements but also as high-power, high-temperature electronic elements. Presently, high-quality GaN crystals can be grown on a sapphire substrate by using the MOCVD growth method.
An example of the principal core techniques is the development of a low-temperature buffer layer. It is possible by using the MOCVD growth method to grow an amorphous or polycrystalline AlN or GaN buffer layer on a sapphire substrate at a low growth temperature of 400° C. to 700° C., and grow high-quality GaN crystals at a high temperature of 1,000° C. or more. That is, the technical development of the low-temperature buffer layer is presently the principal technique reaching the production of light emitting elements.
At present, however, the important subjects of the GaN-based light emitting elements are a high efficiency, a high output, and a short wavelength in the ultraviolet region. GaN-based thin films and thick films can be grown by methods such as MOCVD (Metal Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy), and HVPE (Hydride Vapor Epitaxy), in accordance with the purposes of the growth, and optical elements or electronic elements are implemented by using these methods. In particular, the HVPE growth method is mainly used in the fabrication of a GaN freestanding substrate obtained by growing a thick GaN film on a sapphire substrate at a high growth rate of 100 μm/hr or more, and separating the substrate and thick GaN film by the laser lift-off method. In the fabrication of GaN-based optical or electronic elements, sapphire or SiC is presently mainly used as a substrate for crystal growth. However, a large lattice mismatching and a large thermal expansion coefficient difference cause a high defect density, that is, a dislocation density of about 1010/cm2, thereby posing many problems such as element characteristic deterioration and the difficulty in element processing caused by the chemical resistance characteristic. Low-defect thin films can be grown by using various buffer layers to decrease the dislocation density, and by using the selective growth or lateral growth technique such as LEO (Lateral Epitaxial Overgrowth) or the PENDEO epitaxy method (non-patent reference 1). However, these growth techniques increase the unit cost of production because a number of steps are necessary to fabricate a substrate before the growth, and also have problems in reproducibility and yield.
A practical conventional technique of fabricating a high-luminance, high-output, blue light emitting diode will be described below. When fabricating a GaN-based light emitting diode on an insulator sapphire substrate, the TOP emission LED method as shown in FIG. 1 that emits light in the direction of the upper portion of a thin film is conventionally mainly used. Recently, however, as shown in FIG. 2(a), the light emission output can be increased to be about twice that of the conventional TOP emission LED method by using the LED-chip method (or the flip-chip method) that emits light in the direction of a sapphire substrate. Also, a high heat dissipating effect can be obtained because it is possible to perform a packing step in which a submount 110 having high conductivity and a thin film that generates heat are packaged close to each other. The output increases because the LED upper metal electrode does not physically limit the light. Also, as shown in FIG. 2(b), a mirror-coating 180 of the submount 110 can further increase the light emission efficiency.
Recently, as shown in FIG. 3, a new LED structure (FIG. 3(d)) having a top-down electrode is proposed which is fabricated by connecting an LED structure formed by growing a thin film on a sapphire substrate 120 by MOCVD (FIG. 3(a)) to a Si substrate 190 by using a metal junction layer 182 (FIG. 3(b)), and separating the sapphire substrate 120 and thin film by using the laser lift-off technique (FIG. 3(c)).
As another method of a high-output, high-efficiency light emitting diode, a patterned sapphire substrate 122 is sometimes used as shown in FIG. 4. This is a method in which fine patterns formed on the sapphire substrate 122 cause irregular reflection of light generated from an active layer of a light emitting element, and increase the amount of light emitted from the surface by suppressing the transmittance of light through the sapphire substrate, thereby increasing the light emission efficiency of the element.
As described above, the flip-chip technique, the use of the patterned sapphire substrate, the technique that increases the efficiency by using a reflecting electrode metal, and the like have been proposed to fabricate high-luminance, blue and ultraviolet light emitting diodes and laser diodes, but various problems such as the complexity of fabrication steps and the inefficiency of production have arisen. When growing a thin GaN film by using the conventional techniques, it is essential to form a seed layer of a low-temperature GaN or AlN buffer layer in order to grow a high-quality thin film because the growth is hetero growth on a substrate made of different material, such as a sapphire. However, even when this buffer layer exists, a large lattice mismatching and a large thermal expansion coefficient difference cause a high defect density, that is, a dislocation density of about 1010/cm2.
Also, an electrode is difficult to form on the sapphire substrate because the sapphire substrate has an insulation property. Therefore, complicated steps including a step of dry-etching a grown thin film by about a few μm is necessary to form an electrode for a device.
Note that the fabrication of a device on a GaN substrate, instead of a sapphire substrate, has been regarded as most promising in order to greatly increase the LED light emission efficiency, achieve a high-current operation and high luminance, and fabricate a high-output ultraviolet laser. However, GaN bulk growth is technically difficult in the conventional GaN substrate fabrication. Instead, therefore, a thick GaN film is grown on a sapphire substrate by using the HVPE method, and separated from the substrate by mechanical polishing or the laser lift-off method, thereby fabricating a GaN freestanding substrate. Since, however, these methods require a high process cost after the growth of the thick GaN film, the development of a low-cost process has been desired.
The present inventors have grown a GaN layer on a CrN layer directly formed on a substrate by the MBE method (non-patent references 2, 3, and 4). To increase the area and throughput, however, it is possible to stack Cr by a method such as sputtering suited to mass-production, instead of stacking a CrN layer by the MBE method or the like, and form a Cr nitride layer by nitriding Cr in an HVPE apparatus capable of high-speed film formation and mass-production, thereby forming a template for GaN growth. Unfortunately, even when Cr is stacked on a sapphire substrate, this Cr forms a polycrystalline or multi-domain layer. A single crystal is difficult to grow on a polycrystalline or multi-domain layer. In addition, Cr forms an extremely stable Cr oxide (passivity) as is well known (a Cr oxide layer naturally forms on the surface of stainless steel, and protects the interior of stainless steel against corrosion). Since the substrate is moved from the sputtering apparatus to the HVPE apparatus by batch processing, the substrate must be transferred in the air, and Cr surface oxidation occurs during the process. The existence of this oxide layer interferes with the growth of a GaN single crystal. To epitaxially grow single-crystal GaN on a Cr nitride as described in non-patent references 2, 3, and 4, it is necessary to further form the nitrided Cr nitride layer into a single crystal. It is of course also possible to stack another metal by sputtering, nitride the metal, and epitaxially grow single-crystal GaN on the nitrided metal, but the above-mentioned difficulties (a metal film stacks as a polycrystalline layer, causes surface oxidation, and makes the formation of a single crystal difficult) still exist. Accordingly, a demand has arisen for the development of the process of GaN growth on a metal stacked film.
Note that patent references 1 and 2 also describe the growth of a GaN layer on a metal film. However, as the references disclose, although a GaN layer is formed after the formation of AlN, Al is unfavorable to the subsequent GaN growth process because the melting point of Al is low as a metal buffer layer (see patent reference 1). Also, although titanium is used as a metal film to form air spaces in a GaN layer by a Ti film and TiN film and then to detach the GaN layer, the air spaces may deteriorate the crystallinity of the GaN layer (see patent reference 2).
Non-patent reference 1: Pendeo-epitaxy versus Lateral Epitaxial Overgrowth of GaN: A comparative study via finite element Analysis, Zheleva, W. M. Ashmawi, K. A. Jones, phys. Stat. sol. (a) 176, 545 (1999)
Non-patent reference 2: Low-Temperature CrN Buffer Layers for GaN Growth Using Molecular Beam Epitaxy (31st International Symposium on Compound Semiconductors: announced in Sep. 12 to 16, 2004)
Non-patent reference 3: Growth and Characterization of HVPE GaN on c-sapphire with CrN Buffer Layer (31st International Symposium on Compound Semiconductors: September, 2004).
Non-patent reference 4: CrN Buffer Layer Study For GaN Growth Using Molecular Beam Epitaxy (22nd North American Conference on Molecular Beam epitaxy: October, 2004).
Patent reference 1: Japanese Patent Laid-Open No. 2002-284600
Patent reference 2: Japanese Patent Laid-Open No. 2004-39810